Biological Aspects of Sphyraena sphyraena (L., 1758) in the Central Mediterranean (E. Ionian Sea)

Abstract

S. sphyraena is a widely distributed species with low commercial value and no sufficient scientific knowledge of its biology. In the present study, the age, growth, weight–length relationship, otolith morphometry, and reproduction of the species were investigated in the Eastern Ionian Sea for the first time. The von Bertalanffy growth function parameters were L_∞ = 63.65 cm, k = 0.14 year⁻¹ and t₀ = −2.01 years and Φ′ = 2.75. The negative allometric growth in weight was found. Slope b of the weight–length relationship was 2.634. For the otolith moprhometry, the variables radius, length, width, area, perimeter, roundness, circularity, form factor, rectangularity, and ellipticity were examined, which showed that the otolith shape is elongated with an elliptical and rectangular form. Six otolith variables (radius, length, width, area, perimeter, and ellipticity) showed a significant relationship with size. The sex ratio (females/males) was 1:0.74 (no statistically significant difference from 1:1), and the spawning season extended from April to June with peak values of GSI in May for females and April for males. The results of this work improve our knowledge of the species life cycle and provide basic information for species stock identification and fisheries management.

1. Introduction

The European barracuda, Sphyraena sphyraena (Linnaeus, 1758), is a widely distributed species that occurs in the western and eastern Atlantic, Mediterranean, and Black Seas [1]. It is a neritic species found in coastal and offshore waters [2] at depths from a few meters to 100 meters (m) [3]. Small and juvenile individuals form schools, while large adults are frequently observed to remain solitary [4]. As a temperate species, S. sphyraena coexists in the same habitats as the thermophilic congeneric species Sphyraena viridensis [5,6].

S. sphyraena is mainly caught by purse-seines [7]. However, this species is also found in the catches of bottom trawl and trammel nets. Although the European barracuda is considered of minor commercial importance according to the Food and Agriculture Organization (FAO) [1], it may present quite important economic value in local markets [8]. It is also valued as a gamefish [1].

Despite the fact that S. sphyraena is widely distributed throughout many marine environments worldwide, published information on this species is very scarce. The two works from the Atlantic Ocean concern the weight–length relationship (WLR) and the feeding habits of the species [9] and the otolith morphometrics [10]. Restricted information on limited aspects of the biology of this species has been reported in some areas of the Mediterranean basin (Western Mediterranean Sea: otolith morphometrics—[10]; Catalan waters: reproduction—[6]; Algerian waters: otolith morphometrics—[11]; Adriatic Sea: age and growth—[12]; Aegean Sea: diet—[5]; North Levantine Sea: otolith morphometrics—[13]; Egyptian waters: age, growth and reproduction—[8,14]).

The study of biological features is essential to improve our knowledge of the life history of a species, its stock identification, population dynamics, and fisheries management. For the first time, the current study investigates the age, growth, otolith morphometrics (including ten otolith variables), sex ratio, and reproduction of S. sphyraena in the E. Ionian Sea. The goal is twofold: (i) upgrade the scientific knowledge on this species’ life history in the Mediterranean Sea and (ii) provide biological information required in fisheries management in the Greek waters since S. sphyraena constitutes a target species of the National Fisheries Data Collection Programme.

2. Material and Methods

2.1. Study Area and Data Collection

Samples of S. sphyraena (302 individuals) were collected in the Eastern Ionian Sea from 2022 to 2024 in the framework of the Greek National Fisheries Data Collection Programme (Figure 1). Samples were collected during two sampling periods, from April to June and from September to November. All samples examined for the present study were collected as dead individuals in the fishing market due to fishing practices. They were kept frozen for a period before being analyzed in the laboratory for the collection of their biological data (i.e., length, weight, otolith, sex, and maturity).

Each specimen was measured to the nearest mm in total length (TL) and weighed (TW) to the nearest g. Macroscopic analysis of the gonads was performed to determine the sex and maturity stages, which were identified according to the Nikolsky scale [15]. Gonad weight was also recorded, as well as the eviscerated weight (EW) to the nearest g. Pairs of sagittal otoliths were extracted, cleaned, dried, and stored for further examination. Digital images of the otolith were taken at 6.3× magnification under transmitted light against a black backdrop using Image-Pro Plus software (Version 6.0.0.260); each left otolith was placed on the proximal side down on a glass Petri dish filled with water.

Age was based on the reading of annual growth rings, which are presented as alternating opaque and transparent zones along the left sagittal otolith axis from the core to the post-rostrum margin. Readings were reported by three independent readers. When readings of an otolith differed among readers, a second reading was performed in order to come to a compromise. In the case of disagreement, the otolith was excluded. Broken or damaged otoliths were not used in aging and otolith morphometric data collection. The final number of otoliths utilized for age determination and otolith morphometrics were 302 (all samples) and 288, respectively. The date of birth was assumed to be on January 1st, in compliance with the guidelines that most fish age determination laboratories globally follow [16,17]. This assumption is also consistent with the results of the species’ spawning season, which occurs in the first semester of the year [6,14].

The following otolith morphometric variables were recorded based on observations of the left sagittal otolith: otolith area (OA, mm²); otolith length (OL, mm); otolith width (OW, mm); radius (RA, mm); perimeter (PE, mm) (Figure 2); roundness (RD) (the ratio between the actual area and the area of a circle of the same diameter; the factor is larger if and when the shape of the otolith is more circular) [18] taking a minimum value of 1 [19]; and circularity (CI) = (PE²/OA), (compares otolith shape to a perfect circle) [20] taking a minimum value of 4π [19]. Furthermore, the following shape factors were estimated: form factor (FF) = ([4 × OA/PE²]), which is a dimensionless value that indicates the similarity of the otolith contour to a circle, taking values from 0 to 1, with a value of 1 corresponding to a perfect circle; rectangularity (RC) = (OA/[OL × OW]), which gives information about the approximation to a rectangular or square shape, indicating a perfect square if it has a value of 1; ellipticity (EL) = (OL − OW/OL + OW), which reflects the similarity to an ellipse, with values close to 0 indicating a tendency towards circularity [20].

2.2. Data Analysis

2.2.1. Size Composition, Age and Growth

The length (TL) frequency distribution of all samples collected was represented by a histogram using length classes of a 1 cm interval.

An age–length key was constructed based on the otolith age readings of all samples. The Bhattacharya method [21] was performed for the age validation of age readings as applied by FiSAT II software (version 1.2.2 computer program) [22]. The method was used to discriminate normal distribution cohorts in the length frequency of a species that correspond to age groups.

Growth at length was based on the von Bertalanffy equation [23]: L_t = L_∞ (1 − e^−k(t−t0)), where L_t is the predicted length at age t in cm; L_∞ is the mean theoretical asymptotic length in cm; k is the rate at which L_∞ is approached in year⁻¹; and t₀ is the theoretical age at zero length in years [23]. The growth performance index Φ′ (Φ′ = logk + 2logL_∞) [24] was also estimated to compare the growth rate of the species with that of the available published literature.

Growth in weight was based on the total weight–total length relationship (WLR) by applying the following equation: TW = αTL^b, where TW is the total weight in g, TL is the total length in cm, α is the intercept, and b is the slope of the log-linear regression. The eviscerated weight–total length relationship (EWLR) was also estimated by applying a similar equation: EW = αTL^b, where EW is the eviscerated weight in g. The null hypothesis for isometric growth (H₀: b = 3) was examined using Student’s t-test in both relationships as well as ANCOVA, which was used to detect differences by comparing the slopes of the regressions. Differences were considered at the significance level a = 0.05.

2.2.2. Otolith Morphometrics

The mean (±standard error), minimum, and maximum values of the examined otolith morphometric variables were recorded. The relationship of each otolith morphometric variable with TL was also examined by the exponential regression: y = Ax^B (where y is the otolith morphometric variable in mm, x is the total length in mm, A is the intercept, and B is the regression slope). The significance of these relationships was based on the p-value (p-value < 0.05).

2.2.3. Sex Ratio and Reproduction

The sex ratio was estimated as the ratio of females to males. The sex ratio was tested for equality (female/male = 1:1) using the chi-square goodness-of-fit test (χ²). Differences were considered at the significance level a = 0.05.

The study of the reproductive period included two analyses: (i) the gonadosomatic index (GSI) and (ii) the proportion of the maturity stages by month. The GSI was estimated by sex as the mean value of the percentage of the gonad weight to the eviscerated weight of each specimen: GSI = GW/EW100 [25], (where GW is the gonad weight, and EW is the eviscerated body weight). GSI estimation was based on the available data for the months of April to June and September to November. The percentage of each maturity stage was similarly calculated by the sex and month for the available data.

3. Results

3.1. Length Distribution

In total, 302 individuals of S. sphyraena were examined for the present study. The total length of TL ranged between 16.4 and 49.7 cm. The predominant length class was 30.0 cm (Figure 3). For females and males, the TL ranged between 20.3–49.7 cm and 18.5–46.4 cm, respectively.

3.2. Age and Growth

Nine age groups (from 0+ to 8) were identified for S. sphyraena based on the otolith readings (Figure 4). The age–length key presented in

Table 1. Age–length key of Sphyraena sphyraena from the E. Ionian Sea. TL: total length (cm); N: number of individuals.

Sphyraena sphyraena
Length Classes (TL, cm)	Age Classes
	0+	1	2	3	4	5	6	7	8
16.0–16.9	1
17.0–17.9
18.0–18.9	1
19.0–19.9
20.0–20.9		3
21.0–21.9		12
22.0–22.9		5
23.0–23.9		1
24.0–24.9		1	1
25.0–25.9		4	2
26.0–26.9			8
27.0–27.9			10
28.0–28.9			11	5
29.0–29.9			7	13
30.0–30.9			14	12
31.0–31.9			8	10
32.0–32.9				24
33.0–33.9				21
34.0–34.9				13	1
35.0–35.9				14	5
36.0–36.9				15	3
37.0–37.9					19
38.0–38.9					15
39.0–39.9					6	5
40.0–40.9						13
41.0–41.9						2
42.0–42.9						7	1
43.0–43.9							2
44.0–44.9							2
45.0–45.9								1
46.0–46.9								2
47.0–47.9								1
49.0–49.9									1
N	2	26	61	127	49	27	5	4	1
Mean TL (cm)	17.45	22.40	28.84	32.89	37.74	40.90	43.68	46.53	49.70

reveals that the majority of the examined samples belonged to the age group three.

Age validation using the Bhattacharya method showed that the mean length of five groups (cohorts) (I, II, III, IV, V), identified in the length frequency distribution of the examined samples (

Table 2. Mean length (cm) per cohort identified using the Bhattacharya method for Sphyraena sphyraena in the E. Ionian Sea. TL: total length (cm); S.D.: standard deviation; S.I.: separation index.

Group	Mean TL (cm)	S.D.	S.I.
I	22.23	1.17	N.A.
II	27.73	1.15	2.27
III	31.44	2.24	2.02
IV	36.39	2.20	2.03
V	41.04	1.60	2.04
VI	46.56	0.80	2.15

), coincided quite well with the mean lengths of the age groups 1, 2, 3, 4, and 5, derived from the age–length key (Table 1). There were a few individuals with a total length larger than 43.0 cm in our samples, which made it difficult to identify further age groups in this size range.

The von Bertalanffy parameters and the growth performance index Φ′ are displayed in

Table 3. The von Bertalanffy growth parameters of Sphyraena sphyraena in the E. Ionian Sea. N: total number of individuals; L_∞: the mean theoretical asymptotic length in cm; k: growth rate parameter in year⁻¹; t₀: theoretical age at zero length in years; S.E: standard error; Φ′: the growth performance index.

N	L∞ ± S.E (cm)	k ± S.E (year−1)	t0 ± S.E (year)	Φ′
302	63.65 ± 4.05	0.14 ± 0.02	−2.01 ± 0.28	2.75

. The growth curve is shown in Figure 5.

3.3. Weight–Length Relationship

The total weight (TW) of all the examined specimens varied from 18.6 to 399.6 g. WLR for S. sphyraena was found as follows: TW = 0.01355 × TL^2.634 (R² = 95%). The WLR showed that b was statistically significantly lower than three, which indicated negative allometric growth (t-test = −10.63, p-value < 0.01). The relationship between eviscerated weight (EW) with TL was as follows: EW = 0.01172 × TL^2.649 (R² = 95%). According to the p-value of Student’s t-test (t-test = −10.01, p-value < 0.01), the EWLR also revealed negative allometric growth. No statistically significant difference was found in the slope of WLR between TW and EW (ANCOVA, p-value = 0.77).

3.4. Otolith Morphometrics

The mean (±standard error), maximum, and minimum values of the RA (radius), OL (otolith length), OW (otolith width), OA (otolith area), PE (perimeter), RD (roundness), CI (circularity), FF (form factor), RC (rectangularity), and EL (ellipticity) of the left otolith are presented in

Table 4. The mean (±standard error), minimum, and maximum (in parenthesis) values of the otolith morphometric variables of Sphyraena sphyraena from the E. Ionian Sea. RA: radius (mm); OL: otolith length (mm); OW: otolith width (mm); OA: otolith area (mm²); PE: perimeter (mm); RD: roundness; CI: circularity; FF: form factor; RC: rectangularity; EL: ellipticity.

Otolith Morphometric Variable	Sphyraena sphyraena
RA (mm)	5.29 ± 1.15 (2.84–7.73)
OL (mm)	10.10 ± 2.08 (5.61–14.69)
OW (mm)	3.16 ± 0.02 (1.99–4.28)
OA (mm2)	24.90 ± 0.41 (8.44–46.63)
PE (mm)	23.84 ± 4.55 (13.87–33.34)
RD	1.80 ± 0.20 (1.34–3.04)
CI	22.58 ± 2.46 (16.86–38.16)
FF	0.18 ± 0.02 (0.10–0.24)
RC	0.77 ± 0.03 (0.64–0.88)
EL	0.52 ± 0.03 (0.38–0.59)

. The values of RD and CI showed that the otolith shape is quite different than a circle. Moreover, FF was quite close to zero, indicating a non-circular otolith contour. RC showed a value lower than one, which indicated a more rectangular shape, while the EL value, which was significantly higher than zero, indicated a more elliptical shape.

TL presented a statistically significant relationship (p-value < 0.05) with the majority of the otolith morphometric variables examined, except with the otolith shape variables RD, CI, FF, and RC (

Table 5. Parameters of the exponential regression of the total length (TL, mm) with each otolith morphometric variable of Sphyraena sphyraena. RA: radius (mm); OL: otolith length (mm); OW: otolith width (mm); OA: otolith area (mm²); PE: perimeter (mm); RD: roundness; CI: circularity; FF: form factor; RC: rectangularity; EL: ellipticity. A: intercept; B: slope; R²: coefficient of determination; r: correlation coefficient. The p-value of each regression is also shown.

Variables	A	B	R2	r	p-Value
TL/RA	0.02	0.94	0.88	0.94	<0.01 *
TL/OL	0.04	0.94	0.90	0.95	<0.01 *
TL/OW	0.08	0.64	0.85	0.92	<0.01 *
TL/OA	0.00	1.58	0.94	0.97	<0.01 *
TL/PE	0.22	0.80	0.86	0.93	<0.01 *
TL/RD	1.56	0.03	0.00	0.05	0.41
TL/CI	19.57	0.03	0.00	0.05	0.41
TL/FF	0.20	−0.03	0.00	−0.05	0.41
TL/RC	0.77	0.00	0.00	0.00	0.99
TL/EL	0.13	0.24	0.35	0.59	<0.01 *

* Significance level a = 0.05.

).

3.5. Sex Ratio and Reproduction

Among the 302 individuals examined, 15 were unidentified, while the number of identified females was 165 (57.49%) and that of males was 122 (42.51%). The estimated sex ratio of females to males (1:0.74) was not statistically significantly different from the ratio 1:1 (p-value = 0.07).

The study of the GSI index, based on the available data for six months, showed that the highest values were observed between April and June (females: 3.9–8.2; males: 2.3–3.5) for both sexes; the lowest values were observed from September to November (females: 0.5–1.1; males: 0.1–0.3) (Figure 6). The peak of GSI was found in May and April for females and males, respectively.

The analysis of the maturity stages by sex and month indicated that stage 5 (spawning) appeared during April–June in both sexes, with the highest values in May for females and April for males (Figure 7 and Figure 8). Females and males at the early developmental stages (maturity stages 2 and 3) were more numerous from September to November. The smallest spawning female and male S. sphyraena (maturity stage 5) were found at 21.5 and 20.6 cm TL, respectively.

4. Discussion

The life history of S. sphyraena has been fragmentally and sporadically studied to date. The current study aimed to improve the existing scientific knowledge on the species’ age, growth, otolith morphometrics, and reproduction and to provide for the first time original information for the species in the E. Ionian Sea, necessary in stock identification, population dynamics, and fisheries management in the study area.

The results from the otolith readings revealed nine age groups for S. sphyraena. The mean length-at-age of age groups 1–5 coincided quite well with the average length of cohorts I–V estimated by the Bhattacharya length-based method, which validated the ages estimated by otolith readings. A similar number of age groups was reported by [8] for the species in the Egyptian Mediterranean waters, while [12] estimated five age groups in the Adriatic Sea (

Table 6. The weight–length relationship and growth parameters of Sphyraena sphyraena from different study areas (N: total number of individuals; TL range: minimum–maximum total length in cm; age groups: number of age groups range; α: intercept and b: slope of the weight–length relationship; L_∞: mean theoretical asymptotic length in cm; k: growth rate in year−1; t₀: theoretical age at zero length in years; Φ′: growth performance index).

Reference	Area	Sampling Years	N	TL Range (cm)	Age Groups (Range)	α	b	L∞ (cm)	k (year−1)	t0 (year)	Φ′
[9]	SE Atlantic (Ghana)	2011	75	7.2–26.1		0.00670	2.91	-	-	-	-
[8]	Egyptian Mediterranean Waters	1998–1999	627	22.0–51.0	1–8	0.00500	2.92	55.27	0.12	−3.25	2.58
[12]	Adriatic Sea	2020–2021	113	23.4–42.5	1–5	-	-	55.58	0.12	−4.29	2.57
This study	E. Ionian Sea	2022–2024	302	16.4–49.7	0+–8	0.01355	2.63	63.65	0.14	−2.01	2.75

). The mean length-at-age for the first three age groups (1–3) reported in these two studies was quite similar to our results. Differences in the older age groups among these works may be related to differences in the size range of samples and/or variations in the species’ growth patterns across different geographic areas. Nevertheless, the age readings of the older age groups, which in most cases are derived from a low number of individuals, should be treated with more attention.

The von Bertalanffy growth parameters and the growth performance index (Φ′), estimated in the present work for the species in the E. Ionian, were higher than those reported in the published literature, suggesting a higher growth rate in the study area than in other Mediterranean regions (Table 6). This may be the result of differences in the length range of the examined individuals; differences between the years of study as some of the results were derived from data from the 1990s, environmental factors like temperature; and the food availability and/or genetic variability of the species among the study areas.

The study of WLR analysis revealed an allometric negative growth (b < 3) for S. sphyraena. The value of slope b of the WLR was lower than that of other regions (Table 6), indicating that the species growth in weight was lower in the E. Ionian Sea than in other study areas. Factors such as genetics, habitat, changes in the length composition, the number of samples examined, preservation methods, temperature, food habits, sex, maturity stages, sampling period, rates of growth, and age are commonly attributed to the variations in the WLR parameters [26,27,28].

In this study, the otolith shape variables showed that otoliths of S. sphyraena are elongated and more elliptical and rectangular than circular. References [11,13] found RD, CI, RC, and EL values that coincide with the results of the present study, meaning that the otolith shape of the species is more elliptical and rectangular and quite different than a circle. The higher value of FF was estimated by Yedier (2022), indicating a more circular otolith contour for the species in the Levantine Sea than in the E. Ionian Sea. According to [29,30] the shape characteristics of the otolith give information about the ecological adaptations and feeding strategies of fish. Based on these works, our findings regarding the elongated and elliptical shape of S. sphyraena otolith revealed a piscivorous and omnivorous species with better swimming capabilities and the fast processing of sensory information. Knowledge of the species’ diet [5,9] confirmed the piscivorous character of the species and its good swimming ability.

The values of the otolith moprhometric variables OL and OW found by [12]) for the species in the Adriatic Sea were quite similar to the values of the present study. However, those mentioned by [13] were lower than those for the E. Ionian. This may be due to the smaller size range of the samples of the former study. Moreover, differences in otolith morphology could reflect differences in diet [31], ontogeny [32], fish ecology-related factors, including substrate type [33], and environmental variables, such as depth and temperature [34], which are factors that can play a role in the observed fluctuations, highlighting the intricacy of otolith development in response to environmental changes.

The S. sphyraena otolith variables RA, OL, OW, OA, PE, and EL examined in the present work showed a statistically significant correlation with TL. Similar findings for the first five variables were mentioned by [12,13]. There is no study that compares the relationship of the otolith shape variables RD, CI, FF, RC, and EL with the TL. Our results indicate that the otolith shape of the species becomes more elliptic with increasing TL.

Regarding the sex ratio, our results were similar to the findings of [14] and [12] for the species in the Egyptian waters and the Adriatic Sea, respectively. The reproduction period found for the species in the E. Ionian Sea coincided with the period reported for the southern and western Mediterranean waters [6,14]. This coincidence supports the validity of our results, although the gaps in the sampling months of the present work may raise questions about the study of reproduction for which monthly samples are generally required.

The current study provides significant scientific knowledge on the age, growth, otolith morphometrics, sex ratio, and reproduction of S. sphyraena in the eastern Ionian Sea for the first time. This information improves knowledge of the species’ life history, providing useful information on biology, ecology, and behavior, as well as stock identification, population dynamics, and the fisheries management of this species.

In the context of global climate change, the findings from this study are particularly important. Changes in sea temperature, salinity, and other environmental conditions can have profound effects on marine species. By understanding the age, growth, and reproductive patterns of S. sphyraena, we can better predict how this species might respond to the shifting of environmental conditions and what that means for its future sustainability. Moreover, since the increase in temperature may support the thermophilic species S. viridensis more, any information related to S. sphyraena and the interactions between these two congeneric species are of great importance.

Furthermore, the insights gained from this study are critical for regional fisheries management. Effective management strategies rely on accurate data regarding species populations and their dynamics. This study provides essential baseline data that can support the assessment of the stocks of the species, particularly in the E. Ionian Sea.

To further refine our knowledge, additional studies on the species’ otolith microstructure, genetics, and population dynamics, as well as environmental conditions, are necessary. These future studies may help better explain climate change and its impact on fisheries management.

5. Conclusions

The information on S. sphyraena age, growth, otolith morphometrics, sex ratio, and reproduction has been updated by this work by examining samples of this species from the Eastern Ionian Sea for the first time. Nine (0+–8) age groups were identified in the present study with von Bertalanffy growth parameters as follows: L_∞ = 63.65 cm, k = 0.14 year⁻¹, and t₀ = −2.01 years. A negative allometric growth in weight characterized the species. The examined otolith variables showed an elongated, elliptical, and rectangular shape. Six otolith variables (radius, length, width, area, perimeter, and ellipticity) exhibited a strong correlation with size. The sex ratio was close to 1:1. The peak of the reproduction period of S. sphyraena was found in May for females and April for males.